1. Field of the Invention
The present invention relates to an image pickup apparatus that varies optical properties that is arranged in an endoscope, an electronic endoscope including the image pickup apparatus, and a lens unit in the image pickup apparatus.
2. Description of Related Art
As is well known, electronic endoscopes are widely used for observing and/or treating the inside of (body cavities of) the human body or for examining and/or repairing the inside of industrial plant facilities and the like. In recent years, in some electronic endoscopes, an image pickup apparatus is used that is equipped with a focus function and a zooming function that focus and zoom in on a photographing object by moving an observation optical system in the optical axis direction for photographing.
More specifically, when carrying out endoscopic observation, an apparatus is desired that makes it possible to change optical properties such as the depth of focus, the image forming magnification ratio, and the viewing angle with respect to an observation target region in accordance with the observation site or observation purpose or the like. Recently, an image pickup apparatus (image pickup unit) is known that is configured to enable movement of one or a plurality of optical lenses among an objective lens group included in the image pickup apparatus (image pickup unit) in an optical axis direction, to thus enable adjustment and alteration of the optical properties.
This kind of technology that varies a lens frame for a focusing function of an image pickup apparatus is disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 5-341209. The aforementioned Japanese Patent Application Laid-Open Publication No. 5-341209 discloses technology that moves a lens frame by fixing one end of a coil spring formed by a shape memory alloy (referred to as “SMA” hereinafter) wire to a protruding portion that is integrally formed in a lens frame to which a lens is attached and passing or not passing a current to the coil spring through two lead wires connected thereto.
Further, for example, an endoscope disclosed in Japanese Patent Application Laid-Open Publication No. 2007-229155 utilizes an urging spring and SMA wire as an actuator apparatus for moving forward and backward a movable lens frame that retains a movable lens that varies optical properties to enable a reduction in the size of an image pickup unit having an actuator and thereby reduce the diameter of the distal end portion of an endoscope insertion portion. According to this endoscope, the movable lens frame is moved backward against the urging force of the urging spring when the SMA wire is caused to contract by passing a current thereto, and thereafter when the current being passed to the contracted SMA wire is stopped to cause the SMA wire to expand, the movable lens frame is moved forward by the urging force of the urging spring, to thereby varies the optical properties.